Change the YAML serialization format

[felipecsl]

Summary

This allows us to use a serialization format that looks more readable and more similar to what Betamax does. It also dumps the body as as plain text instead of base64 if it's deemed plain text (media type is either text/* or application/*).
Unfortunately SnakeYAML does not support immutable types well, so we need to use these javabeans style objects in order to have the YAML serialized the way we want.

Pending:

• Remove javabeans setters if they are not needed
• Fix remaining tests
• Update existing tapes